We are investigating the developmental program in highly differentiated cells, specialized for production of a few specific cell proteins. These are the silkmoth follicular cells, which produce a proteinaceous eggshell (chorion). We have attempted to analyze differentiation in these cells quantitatively, by measuring rates of specific protein synthesis during development and analyzing the rates in terms of possible controls at several levels of information flow (gene multiplicity, transcription rates, messenger stability, and translation efficiency). The current emphasis on this work is on the DNA level: we are attempting to characterize the chorion genes, and to study their arrangement in the chromosomal DNA, bearing in mind the regulatory as well as evolutionary implications of structural gene arrangement in chromosomal DNA. BIBLIOGRAPHIC REFERENCES: Kafatos, F.C., 1976: Sequential cell polymorphism: A fundamental concept in developmental biology. Advances in Insect Physiology 12, in press. Hakim, R.S. and F.C. Kafatos, 1976: Cellular metamorphosis II. The larval labial gland duct and its prospective adult fates in the tobacco hornworm. Develop. Biol., in press.